This invention relates to time division or other switching or key telephone systems and more particularly, to such systems employing resonant transfer techniques.
Resonant transfer as disclosed for example, in W. D. Lewis U.S. Pat. No. 2,936,337 issued on May 10, 1960, involves the use of an extra series inductor to resonate with the sampling capacitor of a port circuit during the interval that the port circuit's sampling switch is closed so that all of the voltage on the sampling capacitor will be transferred through the switch during an interval of time equal to any odd number of half cycles of the resonant frequency.
In W. B. Gaunt U.S. Pat. No. 3,319,005 issued May 9, 1967, a circuit is shown which employs resonant transfer techniques to permit a number of station ports to confer with each other. The voltages appearing on the sampling capacitors of each of the station ports are interchanged during successive time slots with the voltages on a sampling capacitor in the conference port.
In British Pat. No. 904,231 published Aug. 22, 1962, a time division system is shown in which each port circuit has three gates associated respectively with the receiving path, the sending path and the bidirectional path to the telephone set. The gates are operated during different time slots to permit the bidirectional exchange of the voltage samples and the use, if desired, of one-way amplifiers in each of the one-way transmission paths.
More recently, D. G. Medill and P. A. Vachon in U.S. Pat. No. 3,835,259 issued Sept. 10, 1974, have shown that a useful conferencing system may be provided without using resonant transfer techniques. The Medill-Vachon system uses the sample and hold techniques and equips each of the port circuits with a pair of operational amplifiers having different gains available at different ones of their respective input terminals.
While each of the foregoingly described switching systems is of utility in its own right, the need persists to provide a conferencing arrangement that has good return loss at the talking station, suitable attenuation in the voice path among the locally conferring stations and as little attenuation as possible, in view of stability requirements, in the voice path between each of the stations and the external or central office lines.